Mercury barometer at Chastleton House

Chastleton House is a wonderful Jacobean manor house between Chipping Norton and Stow on the Wold. It's charm is that it has hardly been altered over the years because the family ran out of money. I found this mercury barometer. You make them by getting a tube about a metre long with one end closed. You fill it with mercury. You cover the open end and invert it into a tub of mercury. Air pushes down on the open tub of mercury but not enough to support a metre of mercury, so the mercury sinks down the tube. It settles to a height supported by air pressure and does up and down a bit as air pressure varies. As befits an old house, the scale is in inches. But I was really impressed by the Vernier attachment. The top of the attachment points to a reading of 29.6 inches. You then go down the Vernier until its scale divisions line up with those on the inches scale. That's 2 scale divisions, which means that we can say more precisely that we have 29.62 inches of mercury. I'm interested that it was done in tenths of an inch. *For the record - mercury vapour is poisonous so don't make you own ....!*